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One "rule of thumb" that is often given out by writing centers and tutors (for example, see the Yale Center for Teaching and Learning) is that anything in a general-reference print encyclopedia (e.g. World Book, Encyclopaedia Britannica, etc.) is "Common Knowledge" and never (or, at least, hardly ever) needs a citation in academic writing as facts do not get added to encyclopedias until they become generally recognized in the scientific community as true. The major exception to this is that one must cite if one quotes the exact text of an encyclopedia.

The Yale link above explains that Common Knowledge "...is knowledge that most educated people know or can find out easily in an encyclopedia or dictionary."

To what extent is this actually true?

  • Is this mostly true with a few exceptions?
  • Is it partially true and partially false?
  • Is it almost entirely false?

To be clear, I am asking about general principles and best practices in citation, not any specific context. Contact your editor or instructor.

The statement can be true even if there are minor exceptions. For example, "This is true, except that orbital periods of celestial bodies must always be cited because SCIENCE!!!11!111!one, even if they were first measured hundreds of years ago and verified hundreds of times to within 0.1% of their accepted value today" or "This is really true 99.9% of the time, but there was that old case where an editor of the Encyclopaedia Britannica 1990 had a momentary lapse of professional judgment and allowed a controversial statement on the mating habits of Bactrian camels to be printed despite the fact that it had been first published in 1988 and lacked consensus in the scientific community at that time" (I'm not saying that this actually happened, it's just a hypothetical example).

This question is about general knowledge that constitutes Common Knowledge across fields. There is a concept of field-specific Common Knowledge, e.g. where Boyle's Law is considered common knowledge in Chemistry and Physics and needs no citation but might require a citation if used, for example, in a Gender Studies paper. This question is not about such domain-specific Common Knowledge.

In response to comments, I am using the term "Common Knowledge" in its sense as a term of art in Academia for knowledge that need not be cited because it is sufficiently old, sufficiently well-reproduced, and/or sufficiently non-controversial that a citation would serve little, if any, practical purpose, not the sense of knowledge that most people on the street have memorized. For example, the fact that the typical habitat for the Venus' Flytrap consists of subtropical marshes of the US East Coast is Common Knowledge - the average person you meet on the subway or at the bar might or might not know this, but any decent encyclopedia will explain it to you, and you would be hard-pressed to find a botanist (or any scientist, for that matter) who would seriously challenge it.

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    Could you give an example of this rule-of-thumb instruction, i.e. a link or something? Haven't come across that yet, would be happy to see the source. – user153812 May 3 '18 at 15:46
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    @user153812 sure, see here : ctl.yale.edu/writing/using-sources/… "...it is knowledge that most educated people know or can find out easily in an encyclopedia or dictionary." – Columbia says Reinstate Monica May 3 '18 at 15:57
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    Prof Feynman in one of his lectures on thermodynamics said he got most of his material from the encyclopaedia ... but that is high level... good lecture though !! – Solar Mike May 3 '18 at 16:33
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    Given that over half of (U.S.) survey respondents couldn't identify which half century the American Civil War occurred in, I'm not sure what is meant by "Common Knowledge". – shoover May 3 '18 at 20:02
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    @RobertColumbia so your question becomes so broad is there a chance of a sensible answer? – Solar Mike May 3 '18 at 20:11
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In addition to the good answers already posted, I would suggest that a better way to interpret the contents of an encyclopedia is not as "common knowledge" but as "generally accepted knowledge."

Under this interpretation, it becomes clear that "if you found a statement in the encyclopedia, you don't have to cite it" is not reasonable, as the encyclopedia can be viewed as a sort of mega-review paper. Along those same lines, one will generally find that (as one would expect in a review) the encyclopedia itself is likely to be citing sources for the more specific or specialized information that it contains.

However, one can nicely invert the concept as a rule of thumb: "If you think you don't have to cite a statement, you can confirm by checking an encyclopedia."

In short: if an encyclopedia agrees with you that a citation is no longer necessary, then you should generally feel safe in not adding a citation.

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Taking the statement “anything in an encyclopedia is common knowledge” at face value reveals it is obviously false. In the comments it was pointed out that half of Americans don’t know what century their civil war was in, but there’s also the fact that two thirds of people worldwide have either not heard of the holocaust or engage in holocaust denial. I recently was in a regional trivia competition with thousands of contestants where I had to explain to the organizers that one of their questions were wrong because they had confused density and mass. Virtually all of the questions on Jeopardy! are answerable if you knew everything in an encyclopedia, but even the specially trained contestants don’t know all the answers. Based on these examples alone, I think the idea that the majority of people know all the facts in most encyclopedias is nonsense.

I would strongly advocate for the idea that encyclopedias are irrelevant to the question of if something should be cited. Besides the fact that the vast majority of people don’t know most of the facts in an encyclopedia, there are other reasons to cite things, including:

  1. Giving credit to the discoverer
  2. Providing a link between your work and previous work on the subject
  3. Making it easier for readers to find similar relevant articles.
  4. Providing accountability for your positions. Sometimes encyclopediae are wrong, and the process of looking for a source would hopefully reveal your mistake.

Even if we pretended that it was common knowledge that every triangle-free graph contains an independent set of size Ω(sqrt(n log n)), all the above reasons to cite that fact would apply.

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    Also, if everyone knew the contents of encyclopedias there would be no need for them to exist in the first place... – Stella Biderman May 3 '18 at 21:52
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    I think what is meant by common knowledge isn't something that everyone knows, but I've that researchers in the fields should be reasonably expected to know. – user0721090601 May 3 '18 at 21:57
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    nitpick: tungsten's only the 5th-densest naturally occurring element, behind rhenium, platinum, iridium, and osmium. – Geoffrey Brent May 4 '18 at 1:07
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    @StellaBiderman the top four are all stupendously expensive, so perhaps it was some variant on "densest practical option for purpose X"? – Geoffrey Brent May 4 '18 at 1:50
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    I feel like I should have known that every triangle-free graph contains an independent set of size Ω(sqrt(n log n)), but then I remembered that I don’t have to because Wikipedia knows it for me. – JeffE May 4 '18 at 2:46
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I'm glad the original poster linked to the Yale Center for Teaching and Learning advice about this, because I could not initially believe this was a standard being put forth.

I might believe that the content of the first paragraph of a general encyclopedia entry might be common knowledge. But the purpose of a citation is to point the user to a source for further knowledge, and/or to allow them to verify that you are reporting accurately. (Stella Biderman's answer is very nice as to the purpose of citing.) Particularly when giving guidance to students, I encourage citing resources that are on the border of being general knowledge, especially if those students are prone to "forget" to cite more generalist sources from which they learned overviews and opinions of a topic, which are sometimes very blatantly echoed in the paper.

Encyclopedias feature useful generalizations, as well as specific facts. I would not require a citation for, "Gold mining is an important part of Mexico's economy," but I would like a citation for, "Mexican mines produce 3.2 billion tons of gold annually." (I completely made up this figure... but it would be nice in such a case to be able to check the source's usage of both "billion" and "ton", whether the measurement is of ore or refined gold, and what year the figure pertains to.)

Why might there be this "encyclopedia" guideline for common knowledge? There are times you don't want writing weighed down by footnotes. In journalism, each claim is (ideally) fact-checked prior to publication. Perhaps in peer-reviewed journals, the theory is that uncited facts are confirmed by the peer reviewers, who would point out that they need a citation if they are not common enough. And perhaps in many rhetoric classes, students appeal to truly common knowledge often enough that instructors do not want them to clutter up their writing with citations for the banal ("America's founding documents proclaim the importance of liberty"; "Facebook is a daily staple for the typical college student").

Finally, I'll argue that the process of confirming "common knowledge" can be generative and improve the paper. When I've sought to use common knowledge in the introduction of my papers, I often looked things up to confirm the key details. (I'm great at trivia, but I "trust, but verify" if I put something in writing.) When I do this, I sometimes stumble across an even more telling anecdote or specific detail that furthers my point, or a helpful resource that explains a core topic in a way students might appreciate. Sometimes I find that the original paper that put forth a now common idea also contains highly relevant extensions that have not been widely developed.

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    I feel like there's a huge difference between introduction and the other parts of a paper. While in the introduction I add a citation to most statements (except if it's undergraduate textbook knowledge) while in the main part you go away from giving a citation to basically everything otherwise a 4 page communication would need 10 pages of references. I mean you wouldn't cite a math textbook about the least squares method if you use it... – DSVA May 4 '18 at 7:26
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    @DSVA Good point! Just imagine how many citations Gauss/Legendre would get if every paper using OLS had to cite them... – cactus_pardner May 4 '18 at 19:09

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